Transformer cores and method of making same



June 27, 1967 w, OLSEN 3,328,737

TRANSFORMER CORES AND METHOD OF MAKING SAME Filed March 30, 1965 2 Sheets-$heet 1 INVENTOR WILLY OLSEN BY flaw/- ATTQRNEYS June 27, 1967 w, OLSEN 3,328,737

TRANSFORMER CORES AND METHOD OF MAKING SAME Filed March 30, 1965 2 Sheets-Sheet 2 INVENTOR WILLY OLSEN ATTORNEYS United States Patent 3,328,737 TRANSFORMER CORES AND METHOD OF MAKING SAME Willy Olsen, 238 E. Main St., Mount Vernon, Ill. 62864 Filed Mar. 30, 1965, Ser. No. 443,929 11 Claims. (Cl. 336211) The present invention relates generally to formed transformer cores and a method of making same and, more particularly, to a formed transformer core having excellent magnetic characteristics and which may be readily and economically assembled.

In a standard form of inductive apparatus or, more particularly, transformer construction, a coil structure is provided with a coil window having a magnetic core passing therethrough and enclosing at least one leg of the coil. The core in this type of structure is normally assembled from a plurality of groups of laminations to provide a generally rectangular or square core configuration. In discussing such a core, the term legs refers to the core member or members passing through the window in the coil and the member or members parallel thereto. The term yokes refers to the members joining the legs.

In one general form of core construction, the ends of each lamination are butt jointed, with all of the joints lying in the same leg. In manufacture of the core, the individual laminations are assembled in groups, the groups are nested within one another and the structure is then shaped and heat treated to relieve stresses in the material. The groups of laminations are then separated and individually assembled on the coil with the joints located in the coil window. Ellis Patent No. 3,107,415 provides an example of such a device.

With the aforesaid type of structure, certain difiiculties are encountered in the magnetic area; that is, with the magnetic efficiency of such cores. Specifically, due to the fact that all of the joints of the laminations lie along one leg of the core and all of the joints are of the butt type and perfect abutment cannot be achieved, an air gap exists across the entire core which tends to reduce its magnetic efliciency particularly where overall high space factors are employed. Further, the core assembly procedeure of expanding the various laminations to fit them into the coil window develops stress in the core material, particularly at its corners, which stresses increase the magnetic losses in the circuit.

In addition, in the prior art method of construction where the laminations are all fitted onto the core such that the butt joints of the laminations are positioned in the coil window, a certain degree of difiiculty is encountered in properly positioning the innermost group of laminations. Specifically, the first group of laminations, when placed on the coil are normally employed as a core form about which the remainder of the laminations are assembled. It is often ditficult to accurately align the ends of the various laminations and thereafter to hold them in this position during the assembly of the remainder of the groups of laminations. Various solutions to this problem have been suggested, but none of them have been completely satisfactory due to the fact that such procedures normally increase the residual stresses in the innermost group of laminations as a result of the assembly process.

In accordance with the present invention, the magnetic properties of a core employing butt-jointed laminations are improved by employing several novel concepts. One of these concepts relates to the fact that the laminations in each group of laminations have their butt joints arranged in a zig-zag pattern which advances about the core in a given direction. Thus, for instance, each group of laminations may include ten distinct laminations and the butt joints of two of these groups form a V. The butt joints on one leg of the V are unaligned with the ice butt joints on the other leg of the V and the adjacent leg of an adjacent V-shaped configuration of joints. Also, the apices of the VS in each of the groups of laminations are advanced in one direction about the core relative to the apex of the immediately preceding V-shaped group of joints. In this manner, there is no continuous air gap across a leg of the core so that the air gap due to each individual butt joint is surrounded above and below by a continuous magnetic path which tends to minimize the leakage at each point. This arrangement increases the magnetic eificiency of the core but does not reduce magnetic'losses due to the fact that there is no continuous magnetic path around the core since each lamination has an air gap at the butt joint.

In accordance with a second feature of the present invention, at least one lamination in each group of laminations is overlapped; the overlapped joints, due to the overall arrangement of the laminations, being staggered relative to all other overlapped joints. Thus, the usual objection to overlapped joints that the Width of the core is greatly increased is obviated. On the other hand, a continuous magnetic path about the core is provided for each group of laminations. It has been found that this structure, which minimizes the leakage flux generated at each butt joint and which further provides a continuous magnetic path through at least one location in each group of laminations increases the magnetic efiiciency of the core by several percent. Also, the overlap of one lamination in each group simplifies the assembly procedure since the overlapped lamination aligns all of the butts-jointed laminations which cannot be easily accomplished where all butt joints are employed.

In accordance with another feature of the present invention, the residual stresses in the magnetic cores resulting from bending of the laminations during assembly about the coil are reduced by reducing the space factor of the core at the corners and along the end yokes of the core. It has been found that, although it was previously considered that the space factor of the core should be as high as possible in both the legs and the yokes of the core, it is only essential to maintain a very high space factor in the legs of the core. By reducing the space factor in the end yokes of the core, the residual stresses at the corners of the core can be reduced due to slippage of the laminations relative to one another during both heat treating and assembly of the device, thereby reducing the stresses at the corners of the core. It has been found that so long as the space factor in the legs is maintained as high as possible, which is normally between 96 and 99 percent, lowering the space factor in the region of the corners and the yokes of the core to between and percent, a several percentage reduction in losses due to core stress can be effected.

It is an object of the present invention to provide a formed-type transformer core construction and method of making same wherein the core has superior magnetic characteristics and is capable of rapid, accurate and economical assembly.

It is another object of the present invention to provide a formed-type transformer core construction in which the core is built up from a plurality of groups of laminations wherein all of the laminations but one of each group are butt jointed and at least one of the laminations of each group is overlapped.

It is still another object of the present invention to provide a magnetic core construction for transformers and similar electromagnetic devices and method of making same wherein the core is fabricated from a plurality of groups of laminations and wherein each group of laminations has at least one overlapped lamination joint and in which the overlapped joints of the various groups of laminations are off-set relative to one another.

It is yet another object of the present invention to provide a magnetic core construction for inductive devices wherein the core is fabricated from a plurality of groups of individual laminations, most of the laminations being butt jointed with the butt joints being arranged in a zig-zag configuration so that radially adjacent joints are off-set from one another and wherein each group of laminations includes at least one overlapped joint as opposed to the butt joints.

It is still another object of the present invention to provide a magnetic core construction for inductive devices wherein the core comprises two straight legs and two curved end yokes wherein the space factor of the end yokes is less than the space factor of the two straight le s.

Another object of the present invention is to provide a magnetic core construction for inductive devices wherein each core includes two straight legs and two end yokes, the straight legs having a space factor of 96 to at least 99 percent and the end yokes being curved and having space factors together with the corners of the core of 90 to 95 percent.

It is yet another object of the present invention to provide a magnetic core structure for inductive devices which core structure is fabricated from a plurality of groups of laminations and wherein the joints of the laminations of the innermost group of laminations are formed along an end yoke of the core structure so as to serve as a core assembly form, and wherein the remainder of the butt joints of the other groups of laminations are formed on a leg of the core which is to be fitted into a window of the coil associated with the core in the inductive structure.

The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a view in elevation of a core fabricated in accordance with the teachings of the present invention;

FIGURE 2 is an enlarged view illustrating the joint arrangement of one group of laminae;

FIGURE 3 is an illustration showing the relative lengths of the various laminae of a complete core group and a few laminae of the next group of laminae;

FIGURE 4 is a view of a single group of laminae during one of the steps in the formation of the core;

FIGURE 5 is a diagrammatic illustration of the method of forming a core with curved end yokes; and

FIGURE 6 is a cross-sectional view of a magnetic structure during the final stages of assembly.

In the several figures, like elements are denoted by like reference numerals.

Referring now specifically to FIGURES 1 and 2 of the accompanying drawings, there is illustrated a magnetic core, generally designated by the reference numeral 1, fabricated in accordance with the teachings of the present invention. The core is formed from a plurality of internested groups of laminations which, for purposes of explanation, are designated as 2, 3, 4, 5 and 6, with the core group 2 designated as the innermost group of laminations and group 6 being the outermost group of laminations. The various groups of laminations all have the same basic configuration and comprise a pair of parallel legs 7 and 8 joined by curved end yokes 9 and 11. The legs 7 and 8 have a space factor of between 96 and 99 percent, while the curved yokes 9 and 11 and the corners joining the yokes to the legs 7 and 8 have a space factor of approximately 90 to 95 percent. For purposes to be described subsequently, it has been found desirable, though not essential, to maintain the radius of curvature of the yokes 9 and 11 equal to approximately two times the distance between the innermost lamination of the opposing legs 7 and 8. Expressing this mathematically, with R equal to the radius or curvature of the yoke and A equal to the distance between the innermost laminations of the legs 7 and 8, R is equal to 2A.

Referring now specifically to FIGURE 2 of the accompanying drawings, there is illustrated in detail the arrangement of the laminations in two adjacent core groups. As illustrated in the drawings and for purposes of explanation only, the core groups each comprise ten individual laminations. The innermost lamination of each group is designated by the reference numeral 12 and the outermost lamination of each group is designated by the reference numeral 13. In the arrangement illustrated, all of the laminations of the groups 2 and 3 are butt jointed except the outermost laminations 13 which are provided with an overlapped joint. It is known that when butt joints are employed a true butting relationship is never attained, and small air gaps, such as air gaps 14, illustrated in FIGURE 2, exist between the opposite ends of each lamination. The air gaps 14 of the individual laminations; that is, the butt joints of each lamination is olfset relative to the butt joints of its two adjacent laminations in the core group. The oif-set of the joints of the core 2 of FIGURE 2 are staggered from left to right as one proceeds outwardly from the innermost laminations 12. As previously indicated, a zig-zag pattern of joints is employed, and thus the joints of the second group of laminations extend from right to left when proceeding from the innermost lamination 12 to the outermost lamination 13. However, the joints 14 of the group of laminations 3 are off-set radially (transversely to the length of the leg) relative to the joints of the group of laminations 2. More particularly, the joints of the laminations of group 3 are spaced half way between the adjacent joints of the groups 2. In this manner, the leakage flux at the butt joint of each lamination does not interact or in any way affect the leakage flux or tend to saturate the metal in the region of the leakage flux of the other joints.

As has been previously indicated, the outermost lamination of each group is provided with an overlap joint which thus provides at least one continuous magnetic path about the core for each group of laminations. Normally, the objection of the use of overlapped joints is that such a structure increases the size of the core due to the double thickness of metal present at each overlap. However, as can be seen by reference to FIGURE 1 of the accompanying drawings, each of the overlapped joints lies along independent transverse path through the core leg and thus the increase in thickness at any point due to a single overlap of one lamination out of a much larger number is so small as to have no appreciable effect upon the overall size of the apparatus.

Not only does the overlap joint improve the magnetic characteristics of the apparatus, it also renders the construction of the core simpler and increases the speed at which assembly may occur. This latter feature is discussed in more detail subsequently.

Referring specifically to FIGURE 3 of the accompanying drawings, there is illustrated the laminations of one full group and a further partial group of laminations. When cutting the laminations to length, the laminations of each group are cut with a uniformly increasing length to compensate for the increase in length of each lamination as the diameter of the core is built up. The lamination 13 of the group illustrated is increased in length out of proportion to its position in the core so as to provide the requisite overlap. On the other hand, the length of the lamination 12 and subsequent lamination of the nest core group follows the same pattern as all of the laminations of the preceding group except the lamination 13. A dashed line, which is generally designated by the reference numeral 16, has been drawn along the cut surfaces of the laminations of the two groups to illustrate the continuous pattern of lengths, which is broken only by the laminations 13; that is, the outermost lamination of each of the core groups.

In assembling the core, the laminations of each core group are staggered so that when formed into a ring, actually an oval, the joints are offset relative to one another. In this operation, the fact that the outermost lamination is of such a length as to form an overlapped joint facilitates assembly. The overlap of the outer lamination necessarily aligns all other laminations and assures the proper abutment thereof. The largest diameter core group is formed first and is positioned between walls or fixed stops 15. The region of the outermost lamination which overlaps and contacts itself is caused to contact one of the walls due to friction thus produced between contacting surfaces of this overlapped portion, tendency of the core to expand, i.e. uncoil, is greatly reduced. Further, if desired, the overlapped portions of the outer lamination may be spot welded or taped. In any event, any one of these three alternatives reduces the air gaps between the ends of the butt jointed laminations of each group.

The oval configuration of the outer core group results from the set in the stock material from which the laminations are cut. The oval configuration is preferably retained since it increases the initial surface contact between the outer lamination and the wall 15 at the region of overlap of the outer lamination so as to further increase friction in a given configuration.

Each core group is assembled, in order of decreasing size, within the largest, outermost, core group until the desired number of laminations are provided for the particular core being constructed.

It should be noted that the core group adjacent that of FIGURE 4, has its joints offset in a direction opposite to those of the first group, each joint being offset from the joints of the first group. The zig-zag assembly is continued throughout the structure with each leg having its joints offset from the legs on each side thereof.

The core groups may be treated such that each group has a straight line configuration of joints, or alternatively, a zig-zag configuration. Ease of assembly in a given situation will determine the final configuration of each core group. Thereafter, the cores are formed to their desired shape and then heat treated to relieve strains, etc., and to set the core in its final shape.

Specifically, there is placed internally of the core a central forming mandrel 17 which defines the dimension A of the core, dimension A being illustrated and referenced in FIGURE 1 of the accompanying drawings. The core with the mandrel 17 in place is then placed in a structure comprising a bottom forming plate 18 and end forming plates 19 and 21. The forming plates are employed to define the final shape of the apparatus, the plates 19 and 21 defining the outermost expansion of the core as it is pressed into a generally rectangular form and the bottom forming plate 18 providing a surface against which one of the legs 7 or 8 of the final core is to be formed.

A ram, for instance, a hydraulic ram, generally designated by the reference numeral 22, carries a piston 23 on the end thereof. The piston 23 is moved downwardly as illustrated in FIGURE 5 by the ram 22 so as to press against the core and cause it to expand laterally towards,

the walls 19 and 21. The walls 19 and 21 are positioned relative to the overall size of the core such as to engage the yokes 9 and 11 of the core only after the core has been expanded almost to its final form. More particularly, as the piston 23 moves downwardly, and the end portions of the core begin to move towards the walls 19 and 21, the yokes of the core engage the walls 19 and 21 and begin to flatten out. The system is dimensioned such that, when the ram has achieved its final position, the innermost lamination of the legs of the core are caused to lie against the top and bottom sides of the mandrel 17 before the end yokes are completely straightened. By properly positioning the walls 19 and 21, the degree of curvature of the yokes of the core may be determined.

It is apparent that the space factor of the yokes is determined by a combination of their degree of curvature and the lengths of the individual laminations. The indi vidual laminations must be sufficiently long that when the legs are under maximum compression, at which time they achieve their maximum space factor, the length of the laminations permits a certain looseness in the curved yokes which are not compressed during forming. If the walls 19 and 21 are set in too closely, the yokes may be completely straightened and all of the additional material or length of the individual laminations will appear in the corners. However, by permitting the yokes to maintain a curvature of a radius approximately equal to twice the height of the mandrel 17; that is, of the dimension A of the finished core, the extra length in the individual laminae is permitted to accumulate uniformly in the corners and the yokes of the core providing the desired space factor.

The specific dimension of the radius of the yokes as equal to twice the height of the window of the core is not a rigid dimension since, as indicated, the space factor in the yokes and corners may vary from to percent. However, the above formula does define a specific useful working ratio and indicates the general design limitations involved.

It is to be noted that the assembled core, before withdrawal from the assembly apparatus of FIGURE 4, should first be bonded to hold its shape, provided that the outer lamination of the outer core group has not been spot welded or taped.

As indicated previously, once the core is formed by the method diagrammatically illustrated in FIGURE 5, the core is set into a box of the same general size as the finished core and then placed in an oven for heat treating. Thereafter, the core is disassembled into individual core groups. If the outer laminations had been spot welded, the weld is now broken. If the outer group had been taped, the tape would have burned up in the furnace. The individual core groups are now assembled about a coil, which coil may constitute a single coil or multiple coils of a transformer.

Referring now to FIGURE 6 of the accompanying drawings, a coil 29 is provided with two legs 26, 27 as illustrated in cross-section; there being a coil window 28 lying between the two legs, The individual core groups are expanded to open the joints between the laminations so that the core group may be slipped around a leg, such as leg 27 of the coil and then compressed into its desired shaped with the ends of the laminations now butting one another and with the outermost lamination overlapping itself. It is apparent from the drawing that the innermost core group may serve as a form for assembling the remainder of the groups about it. However, since the joints of the innermost group of laminations lie inside the coil window, great care must be taken to assure alignment of the laminations and even then, one cannot be absolutely certain that the laminations are properly aligned with one another or that, once aligned, they maintain proper align ment. By employing the single overlapping lamination of the present invention, it has been found that the ends of the variouslaminations can be more readily aligned since the overlapping lamination acts as a guide. Thus, the assembly operation can be performed more rapidly and with greater certainty than with prior art structures.

While I have described and illustrated one specific embodiment of my invention, it will be clear that variations of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention as defined in the appended claims.

What I claim is:

1. A magnetic core comprising a plurality of groups of curved core laminations, said groups nesting one Within the other, and each of said groups having a plurality of radially superposed laminations butt-jointed on themselves and an outermost lamination of each of said groups having an overlapped joint.

2. A magnetic core comprising a plurality of groups of curved core laminations, said groups nesting one within the other, and each of said groups having a plurality of radially superposed laminations butt-jointed on themselves and an outermost lamination of each of said groups having an overlapped joint; the overlapped joints of each group being circumferentially offset relative to one an other.

3. A magnetic core comprising a plurality of groups of curved core laminations, said groups nesting one within the other, and each of said groups having a plurality of radially superposed, butt-jointed laminations and an outermost lamination of each of said groups having an overlapped joint, the joints of the laminations within each group being circumferentially offset relative to one another and relative to the joints in the adjacent group on each side thereof.

4. A magnetic core comprising a plurality of groups of curved core laminations, said groups nesting one within the other, and each of said groups having a plurality of radially superposed, butt-jointed laminations and an outermost lamination of each of said groups having an overlapped joint, the joints of the laminations within each group forming a zig-zag pattern, and the patterns of each group being advanced about the circumference of said core in a given direction relative to the adjacent inner group of laminations.

5. A magnetic core comprising a plurality of groups of curved core laminations, said groups nesting one within the other and each of said groups having a plurality of radially superposed, butt-jointed laminations and an outermost lamination of each of said groups having an overlapped joint, the joints of the laminations in each group forming a V-shaped pattern with the joints forming each leg of the V-shaped pattern being circumferentially offset relative to the other leg of the pattern of its group and the adjacent leg of the V-sh-aped pattern of an adjacent group of laminations.

6. The combination according to claim wherein the V-shaped patterns advance circumferentially about the core in a predetermined direction as radial location from the center of the core increases.

7. A magnetic core comprising a plurality of groups of curved core laminations, said groups nesting one within the other, and each of said groups having a plurality of radially superposed laminations butt-jointed on themselves and one lamination of each of said groups having an overlapped joint.

8. A magnetic core comprising a plurality of groups of curved core laminations, said groups nesting one within the other, and each of said groups having a plurality of radially superposed, butt-jointed laminations and said core having straight legs with space factors of at least 96 percent and having curved yokes with space factors of between 90 percent and 95 percent.

9. A magnetic core comprising a plurality of groups of curved core laminations, said groups nesting one within the other and each of said groups having a plurality of radially superposed, butt-joined laminations and an outermost lamination of each of said groups having an overlapped joint, said core having straight legs with space factors of at least 96 percent and having curved yokes with space factors of between 90 percent and 95 percent.

10. Themethod of making a laminated magnetic core comprising forming several groups of core members having a plurality of laminations of oriented magnetic strip material of increasing lengths such that the ends of each lamination abut when formed into a ring and each such group having an additional lamination of such a length as to overlap itself when the remainder of said laminations are abutting, said groups being related such as to be capable of tightly nesting with one another, assembling the longest group into a ring with the joints offset from one another, holding said longest group at least at the joint of the outer lamination, assembling the successively smaller groups into closed loops with the joints offset from one another and inserting them inside of the longest group With the overlapping joints offset from one another, inserting a mandrel in said core, placing the core in a press having a side member and two end members, spacing the end members such that when the core is pressed against the side member of the press sufficiently for the core to contact both sides of the mandrel the yokes of the core have been pressed against the end members such as to have a space factor of between percent and percent, and pressing the core against the side member of the press until the core has contacted both sides of the mandrel.

111. The method of making a laminated magnetic core comprising forming several groups of core members having a plurality of laminations of oriented magnetic strip material of increasing lengths such that the ends of each lamination abut when formed into a ring and each such group having an additional lamination of such a length as to overlap itself when the remainder of said laminations are abutting, said groups being related such as to be capable of tightly nesting with one another, assembling the longest group into a ring with the joints offset from one another, holding said longest group at least at the joint of the outer lamination, assembling the successively smaller groups into closed loops with the joints offset from one another and inserting them inside of the longest group with the overlapping joints offset from one another, inserting a mandrel in said core, placing the core in a press having a side member and tWo end members, spacing the end members apart by a distance such that when the core is pressed against the side members of the press sufficiently to engage both sides of the mandrel the parts of the core engaging the end members have a radius of curvature approximately equal to twice the distance between the two sides of the mandrel, and pressing the core against the side member of the press until the space factor of the core in the region of the mandrel is at least 96 percent.

References Cited UNITED STATES PATENTS 2,931,993 4/1960 Dornbush 3362l7 3,186,067 6/1965 Somerville 29155.61 X 3,223,955 12/1965 Olsen et a1. 3362l1 LEWIS H, MYERS, Primary Examiner.

H. W. COLLINS, Assistant Examiner. 

1. A MAGNETIC CORE COMPRISING A PLURALITY OF GROUPS OF CURVED CORE LAMINATIONS, SAID GROUPS NESTING ONE WITHIN THE OTHER, AND EACH OF SAID GROUPS HAVING A PLURALITY OF RADIALLY SUPERPOSED LAMINATIONS BUTT-JOINTED ON THEMSELVES AND AN OUTERMOST LAMINATION OF EACH OF SAID GROUPS HAVING AN OVERLAPPED JOINT. 